15.
(A) about
(B) readily
(C) likely
(D) concerned 

11. (A) bottom (B) cause (C) key (D) fact

12. (A) as much as (B) by all means (C) every so often (D) as soon as

13. (A) promotes (B) is promoted (C) has promoted (D) is promoting

14. (A) is (B) be (C) being (D) have been

16. (A) in two weeks (B) this past week (C) since two weeks ago (D) for over the two weeks

17. (A) Two-third of the adult (B) Most of adults (C) Two-thirds of adults (D) The most of the adults

18. (A) more than (B) as (C) for (D) or

19. (A) problem (B) fact (C) reason (D) likelihood

20. (A) considerable influence (B) preposterous suggestion (C) prophetic wisdom (D) irrevocable decision

複選題21 (A) Duolingo's mission is philanthropic and the approach is genius. (B) It also charges for subscription if users want to block the advertisements. (C) Duolingo is popular for the same reason why Candy Crush and arcade-style video games are successful. (D) What users don't realize is that the texts they are asked to translate during the learning process are later used by real websites. (E) Other language learning tools, Rosetta Stone, for example, have historically been only available to those with the disposable income to fund their learning.